Open Bolt Closed Bolt

I have never really understood these two concepts as applied to semi and full auto. Perhaps someone is willing to clarify?

Closed bolt seems straight forward: cartridge in chamber, bolt forward and locked, firing pin is cocked and falls forward to strike primer when trigger is pulled. In a semi-auto, case is ejected, fresh one chambered, bolt closes,, pin is back in cockes position. More or less?

Open bolt: cartridge enters chamber, bolt remains open, pin does not move independently of bolt. When bolt closes pin strikes primer, bang, bolt opens, case ejects and repeat.
The main diference being the pin does not work independently of the bolt?

Trigger is pulled, bolt comes forward and feeds a round, bolt closes and locks, sear is tripped, round is fired and ejected...process is repeated at a high rate of speed.

Closed bolt: You got that one close enough

mljdeckard

November 15, 2008, 02:37 AM

Right. When you charge a weapon which fires from the open bolt, the chamber is empty, the bolt stays to the rear. The antis think this is an evil feature, since the idea is to not have heat build up in the chamber from rapid firing, and have a live cartridge sitting in it. They think that 'civilian' versions of guns like mini-uzis and tec-9s are safer if they must be made to fire from the closed bolt, with a round in the chamber. I know, good thinking.

If you fire a SAW or a 240, you charge it, and you have to push the handle back forward. when you fire it, the bolt goes forward and chambers the round.

That overexcited former SEAL on Futureweapons demo'd a new squad weapon concept (not really) that did catch my eye. It's essentially an M-16, heavy-barreled, reinforced for heavy sustained fire, designed to fire from the open bolt and a drum magazine. Other than the ammo weight, essentially the same weight and handling as the existing M-16, less conspicuous than a SAW.

Deus Machina

November 15, 2008, 02:44 AM

I think the reason that open-bolt is a 'bad idea' is that it's so much easier to convert to fully automatic.

A closed bolt basically has to be designed as full-auto from the get-go, with a seperate trip or lag on the hammer, or something. An open-bolt semi is actually harder to make than one in full-auto; the difference is a sear that springs back up after the trigger is pulled, and one that is basically a lever and stays down.

No reason to outlaw it, though. Just make it illegal to convert it and slap a harsh penalty on anyone that does.

Blacksmoke

November 15, 2008, 02:50 AM

OK, so as the open-bolt closes, a round is chambered just ahead it. When the weapon is "charged" the chamber is empty.

Now, how does the bolt work in "select fire" weapons like the M14, BAR and Thompson, for the semi-auto setting?

Rubber_Duck

November 15, 2008, 03:00 AM

Some more reading for ya.

http://www.thehighroad.org/showthread.php?t=395792

Loomis

November 15, 2008, 03:14 AM

Open bolt is a bad idea if you are trying to hit the bad guy on the first shot. Immagine if you had a semi auto pistol that always had the slide locked back when not firing. Then when you pull the trigger, the slide releases and fires then then the slide returns to locked back position. It would be kinda hard to keep the sights on the target with the slide slamming forward just before the round fires wouldn't it?

Open bolt is a good idea if you are shooting a machine gun. Everytime you pause, the gun is sitting there with the bolt open. This aids cooling.

BTW, A rifle barrel cools faster when pointed straight up with the bolt open. THe reason being that the barrel turns into a miniature chimney and a natural upward draft is induced that draws cool air in through the opened receiver and out the end of the barrel(top of the miniature chimney)

5knives

November 15, 2008, 04:00 AM

"Open bolt is a bad idea if you are trying to hit the bad guy on the first shot. Imagine if you had a semi auto pistol that always had the slide locked back when not firing. Then when you pull the trigger, the slide releases and fires then then the slide returns to locked back position. It would be kinda hard to keep the sights on the target with the slide slamming forward just before the round fires wouldn't it?"

FWIW,

Generally speaking this is true, yes. But not always, it really depends on the bolt to weapon weight ratio.

My BAR was easily as accurate as any M1 Garand in my platoon on the first shot, Bear in mind that a BAR weighs 17 pounds empty, the bolt is not a major part of that weight. BAR was demonstrated for acceptance by shooting clay pigeons ... thrown clay pigeons, in the air.

M-3 ("Greasegun") on the other hand, with a 2 1/2 pound breech block in a 4 pound weapon was decidedly not especially accurate with the first round. General rule was aim low and let it climb.

Thompson was another heavy son of a gun and not coincidentally quite accurate.

Recoil gets into the accuracy picture too of course, but there was far less felt recoil with a Thompson than with a "Greasegun".

Regards,
:)

7.62X25mm

November 15, 2008, 04:08 AM

The second design "open bolt" is "slam-fire."

Uzi uses slam-fire in the Mod. 45 Carbine and Micro Pistol among others.

Now, how does the bolt work in "select fire" weapons like the M14, BAR and Thompson, for the semi-auto setting?

In the Thompson, it fires from an open bolt. A sear, a lever like piece of metal, tilts down and releases the bolt which travels forward, picks off a round, chambers & fires it, then retracts and this continues until either the mag is empty or the trigger is released. When you move the selector switch to semi, a small finger like projection is cammed upward from just underneath the bolt. This is normally down, and doesn't touch the bolt. In semi auto, the forward traveling bolt hits this "finger" and pushed it forward (it swivels a bit) and this action causes a catch to release the aforementioned sear which rises independently of the trigger now, and when the bolt goes back after firing one shot, the sear is freed, rises, and catches the bolt.
On the other guns you mention, I don't know. You'll have to get someone who knows their operation.

Matrix187

November 15, 2008, 02:41 PM

If you watch a video of a Thompson sub-machinegun shooting you'll see the bolt slide forward before the gun fires. Im pretty sure the Mac-11/10 guns shoot from an open bolt as well.

Blacksmoke

November 15, 2008, 04:03 PM

That open bolt design is some piece of engineering and machining. I am even more impressed with the BAR nad Thompson than ever.

THanks to all of you for the ederkayshun.

eng23ine

November 15, 2008, 09:33 PM

Lets see if im getting this right:

On an open bolt to be in condition 2 you'd have to remove mag, pull trigger to drop the bolt?, then reinsert full mag.

To charge it just pull the bolt back and it will lock in the open position without a round chambered.

pull the trigger and the bolt goes forward, picks up a round out of the mag, and fires it as it hits the bottom of travel?

Then the bolt opens by blow back (or gas) and catches on the sear in the open position.

Trying to see if im picturing this right in my mind.

Rshooter

November 15, 2008, 10:09 PM

M14 - closed bolt. In semi auto the bolt moving rearward cocks the hammer and the sear catches the it before it can fire another round. On full auto the sear is not engaged so it does not stop the hammer after from striking the firing pin again after it returns to battery.

thebucket

November 16, 2008, 02:18 AM

I got to experience first hand today that a Thompson does indeed fire from an open bolt and the recoil even in full auto is not bad. Can't say the same for the recoil of that G3 though.
I was miffed at first when firing the Thompson because the bolt was locked back and I was wondering how to release it to chamber a round. Then it dawned on me. Maybe the Thompson fires from an open bolt. The owner confirmed and I wasted 50 rounds of ammo quite gleefully. :D:D:D

WardenWolf

November 16, 2008, 03:09 AM

Open bolts are also a lot more dangerous due to the possibility of a catastrophic misfeed. That's ultimately what it boils down to: a round may misfeed, but the bolt could still slam down on it and trigger it. And the reduced accuracy / more difficulty in controlling it. It's generally a bad idea for a light non-military weapon.

VINTAGE-SLOTCARS

November 16, 2008, 03:18 AM

There is no doubt that an open bolt gun is dangerous,,but my Mac-10 is awsome. :neener:

AK103K

November 16, 2008, 02:00 PM

Lets see if im getting this right:
You got it right.

Open bolts are also a lot more dangerous due to the possibility of a catastrophic misfeed.
I've shot a lot of open bolt guns, never had a catastrophic misfeed, or any that fired before the bolt was closed.

The safety issues with the open bolts are usually more related to inexperience with them than anything else.

The first and foremost is the apparent condition of the gun when it is in fact ready to fire. Most familiar with closed bolt guns, think the gun is empty and "safe", when in fact, its probably just the opposite. It "could" be empty, if the the mag ran out, then again, you dont know til you look.

As was mentioned, making the gun "safe" can be a problem if you dont know to remove the mag BEFORE you drop the bolt.

Sloppy charging can also be a problem. If you pull back on the charging handle and it slips, even if your finger is off the trigger, the gun can fire. All the bolt has to do is move rearward enough to strip a round off the mag, and its probably going to fire when the bolt goes home. This is also a big issue with dropped or mishandled guns. Most open bolt guns have some sort of lock on the bolt that will help prevent this, but if its not engaged all bets are off. Even when it is, they arent always the surest safeties, depending on the gun.

Another thing people who are not familiar with these type guns need to pay attention to, is the trigger and what to do while shooting and what to do when the gun stops running. If you pull the trigger, it will run until its empty, or you let go of the trigger. You'd be amazed at how many people dont let go. The other issue is, if the gun stops, you have to take your finger off the trigger right away. If you get a hang fire, which can and does occasionally happen, especially if your using questionable, old, surplussed ammo, the gun will stop, bolt down, like its "empty", and then fire unexpectedly when the round decides it did in fact want to fire, a few seconds later. If your finger is still on the trigger, guess whats going to happen next. :)

As far as the accuracy thing goes, once your used to the bolt moving when you pull the trigger, you should have no troubles making good, on demand, first round hits with one. Depending on the gun and its rate of fire, you can also usually get one round "bursts" from most of them. Head shots at 50 yards with something like an MP40 or M45 really are not all that difficult.

unisonic12

November 16, 2008, 03:05 PM

Supposedly, the open bolt design is more reliable with belted/linked ammo.

Claude Clay

November 16, 2008, 03:31 PM

selector set for SAFE or BURST or FULL AUTO.
does this work with closed only or both styles?
and what part of the firing control group does the counting?

Owen

November 16, 2008, 03:39 PM

I was recently shown an open bolt single shot.

the bolt is pulled to the rear to extract and eject the empty.

A fresh cartridge is placed in the chamber. When the trigger is pulled, the bolt slams forward and fires the rifle. It was pretty sweet. Reed Knight has quite a few of them.

Rubber_Duck

November 16, 2008, 03:40 PM

Sloppy charging can also be a problem. If you pull back on the charging handle and it slips, even if your finger is off the trigger, the gun can fire. All the bolt has to do is move rearward enough to strip a round off the mag, and its probably going to fire when the bolt goes home.

This is why the Uzi has a ratcheting sytem where the charging handle isn't allowed forward until it has been pulled all the way back (where it stays locked back until the trigger is squeezed). If you pull it half-way, it stays there until pulled back for the remainder of the charging stroke.

Owen

November 16, 2008, 03:43 PM

Claude,

AFAIK, all 3 rnd burst guns are closed bolt. the counting is done by a ratchet and pawl arrangment. Picture a wheel with three notches in it. One of the sides of one of the notches is much higher than the rest. Each time the hammer falls, the ratchet advances. When the pawl gets to the high spot, the gun stops. When the trigger is released, the pawl is free to ride over the high spot.

rcmodel

November 16, 2008, 03:49 PM

M-14 = On full auto the sear is not engaged so it does not stop the hammer after from striking the firing pin again after it returns to battery.They don't work that way.

The sear works normally whether the selector is set to Semi or Rock & Roll.

There is a trip lever/connector assembly that engages the sear and releases the hammer only after the bolt is closed & locked each cycle.

Claude Clay

November 16, 2008, 03:57 PM

excellent answer, thank you.
now that i know how a gun can count--perhaps you can help me with
another great invention that has perplexed me--this is gun related in that it goes to the hunters refreshment in the field--
a thermos bottle keeps cold things cold :cool: and hot things hot:fire:
how does it know?? :uhoh::scrutiny:

Rubber_Duck

November 16, 2008, 04:06 PM

If you really want to know the ins and outs of how a Thermos works, read this (Google is your friend): http://home.howstuffworks.com/thermos2.htm

unisonic12

November 16, 2008, 04:15 PM

A fresh cartridge is placed in the chamber. When the trigger is pulled, the bolt slams forward and fires the rifle.

What keeps the round from falling out if the barrel is elevated?

Rshooter

November 16, 2008, 04:39 PM

I stand corrected by RCModel, thanks.

Rshooter

Rubber_Duck

November 16, 2008, 04:51 PM

What keeps the round from falling out if the barrel is elevated?

The round isn't in the chamber while the bolt is open.

When the trigger is pulled, the bolt slams forward, stripping a round from the magazine and chambering it, and fires the rifle.

Funderb

November 16, 2008, 04:57 PM

let us not forget the ppsh 41 and its insanely awesome rate of fire.

The main argument against open bolt firearms was that they are not drop safe, well, not as drop safe as closed bolt. Thats is what I had come to understand, I may be wrong.